GENETICS 



251 



if linkage occurs, the genes that were together in the parents go together 

 more frequently than do the other combinations of genes. Thus, as 

 shown in Fig. 198 the genes in the parental germ cells were in one parent 

 Br, in the other parent bR. If linkage occurred between the color of the 

 animal and the direction of its hair, the four kinds of germ cells formulated 

 in the last line of the figure would not be equally numerous. Br and bR 

 would be more numerous than BR and br. A corresponding change 

 in the ratio of Fi individuals would result. The ratio would no longer 

 be9:3 :3 :1. 



Linkage is almost certainly due to the fact that the genes for the 

 linked characters are in the same chromosome. The eight characters 



Fig. 200. — Gregor Johann Mendel, 1822-1SS4, to whom is due much of the present 

 activity in the study of genetics. {Rejnodiiced from ihe repot t of the Royal Horticultural 

 Society Conference on Genetics, 1906. By permission of the President and Council.) 



whose genes are represented in Fig. 189 are thus linked. If the chromo- 

 somes always remained intact, all characters whose genes are in the same 

 chromosome would be absolutely bound together. But such appears not 

 to be the case. Very often, perhaps usually, the chromosomes separate 

 into fragments of greater or less size and these fragments recombine in 

 other ways, so that the association between any two genes in a chromo- 

 some may be broken. Further discussion of linkage would, however, 

 be unprofitable here. 



Gregor Mendel. — The simple operations of heredity described in the 

 foregoing paragraphs are in accordance with what is known as Mendel's 

 Law. An Augustinian monk, Gregor Mendel (Fig. 200), who labored 



